This invention relates generally to a binder or tackifier that dissolves readily in water but then upon drying becomes water insoluble through the formation of covalent crosslinks between polymer chains but thereafter remains biodegradable. In one form of the invention the binder is combined with lignocellulosic or synthetic mulch fiber in an aqueous slurry and sprayed over a ground area where, after drying, an effective erosion-control blanket is created.
There are many known "tackifier" compositions useful for loosely binding together many different kinds of particulate material. Tackifiers are used, for example, in hydraulic mulch applications where a slurry is prepared including fibrous material which is then sprayed over a ground area to perform various functions such as holding the mulch in place. In the same application seed may be included in the slurry or other additives such as fertilizers to promote growth of plants which ultimately control erosion. A dye is usually added, as well, to aid visually in the ground application. One well known tackifier is a water soluble polysaccharide but the problem in their use is that they remain water soluble after the spraying and drying step so they quickly lose their effectiveness after exposure to rain or moist soil. For this reason such sprayed mulches do not perform as well in erosion control as preformed blankets of natural or synthetic fibers.
Another example where water soluble tackifiers have been used is as components of spray systems applied to garbage piles. Such systems, after drying, serve as landfill covers. Again, in the past, these systems have had only poorly defined water resistance due to the failure to chemically insolubilize the water soluble binder component.
Another example of an application of typical known water soluble tackifier compounds was over wood chip piles to generally hold the surface in place to prevent wind erosion. As mentioned, unmodified water soluble polymers are widely used in the erosion control field as binders or tackifiers either with hydraulic mulch or as an overspray on hay or straw that has been spread to control erosion. Unmodified polymers remain water soluble after drying and consequently during the first subsequent rain they redissolve and are generally washed away from the covering and binding function they perform with respect to the mulch fiber and the underlying soil. This renders the resistance of such polymers to wind erosion transient and the resistance to rain erosion is very poor since the polymer survives for only a short time once it rains. Unmodified polymers are exemplified by a family of natural plant "gum" polysaccharides such as guar, locust bean, plantago and the like although some synthetic polymers, for example polyacrylamides are used to a limited extent.
The unmodified water soluble polymers like the natural polysaccharide gums degrade very rapidly in the environment due to biological action. A solution of guar gum and water containing soil bacteria can lose over half its initial viscosity (an indicator of polymer chain length) in only 24 hours. Even in the absence of significant rain to dissolve the polymer, the ground contact life span of unmodified polymers is relatively short as a result of the biological action. It is widely recognized in polymer science that the act of insolubilizing and covalently crosslinking a polymer will retard the rate of its biological breakdown without preventing its ultimate decay into harmless products, principally carbon dioxide and water in this case of a polysaccharide.
Various systems are known in the art for causing the gelation of natural polysaccharide gums in the wet state. Such systems are widely used in the oil well drilling industry for rheology control of drilling fluids. Certain of these systems are based on chemistry involving the heavy metals antimony and chromium (for example, U.S. Pat. No. 3,644,171 discloses the use of the metal based crosslinking agent trademarked "C-20" sold by Steinhall and Company). It is believed gelation in the wet state is brought about by the formation of non-covalent bonds in complexes formed between the metals and the polysaccharide. Certain of these complexes do resist rehydration after drying. However, the heavy metals, antimony and chromium, are not among the elements that are known to be essential for plant growth. The addition of these elements to the soil is therefore undesirable and the presence of "heavy metals" in a tackifier product for use in the environment is widely recognized as being undesirable. Furthermore, the gelation of these systems in the wet state is undesirable from the standpoint of being able to spray the product through a hose from a pump driven hydroseeding machine. Solutions to this undesirable feature have been proposed, for example U.S. Pat. No. 3,712,866 discloses a two-tank spray system whereby the binder or tackifier and the gelling agent are brought together and mixed in the nozzle of the spray apparatus. However, this system requires special equipment that is not commonly available in the hydroseeding industry.
In the erosion-control industry it is known to utilize various latex polymers as alternatives to the natural gum based tackifiers. These polymers are utilized for the same basic functions, that is, to bind particulate matrices together to both stabilize soil and protect a seeding process to enhance natural growth. Usually these latexes are supplied predispersed in water in large barrels or drums which are very heavy and difficult to handle on top of normal hydroseeding machines from which location loading takes place through a hatchway. Splashing and spillage readily occur with these liquid materials. It would be desirable to provide a dry tackifier compound thereby eliminating the use of large barrels or drums on the hydroseeding machine. Another disadvantage in using latexes dispersed in water is that they are very low viscosity compared to the viscous solutions that can be readily prepared from water soluble polymers. The viscosity of the latex as supplied is further reduced when diluted in the water used to apply the material in an erosion control application. These low viscosity latexes when used to bond fibrous mulches readily drain through the mulch fiber and are lost to the soil leaving the mulch relatively unbonded after drying. This loss of binding or tackifying material is an inefficient use of materials and means that a larger quantity of latex polymer is needed to effectively bond the mulches for the intended application simply increasing the cost.
As alluded to earlier premixed dry blends of unmodified water soluble polymers and lignocellulosic fiber are known in the art and used as erosion control materials and examples of such materials can be seen by referring to U.S. Pat. Nos. 4,297,810 and 3,881,278. These materials are supplied to the erosion control industry as prepackaged, easy-to-handle tackified mulches. However, given the binders used, remaining soluble after spraying and drying they lack the longer-term protective properties of the present invention.
In the hydroseeding application for the present invention, the object of using mulch along with the seed is to protect the seed and provide a suitable microclimate while it germinates. The control of erosion on the site is ultimately the result of the plants becoming established. The mulch itself provides for some modest erosion control protection during the germination and growth of the plants. Where conditions dictate more severe erosion potential (steep slopes, heavy rainfall, unstable soils), erosion-control blankets are used which are preformed mats of fibers (for example, straw, coconut fiber, wood shavings and the like held by gluing or stitching to a light web of synthetic fiber) that come in roll form. These materials are rolled out onto the ground that has first been seeded, for example by hydroseeding and held in place by metal staples that are hammered into the soil. These blanket materials are costly to buy and expensive and time consuming to install due to the requirement that a relatively smooth surface be prepared to accept the blanket, the separate seeding step and finally the utilization of staples being hammered into the ground to secure the blanket.
In the landfill application where landfills are periodically covered with a protective layer of a suitable material, one common practice is by using a complete covering of topsoil on a daily basis of about six inches in depth. This is a costly process. An alternative is proposed and disclosed in U.S. Pat. No. 5,082,500 where a spray applied system includes the use of a water soluble polymer, wood fiber combination and certain other additives which will function to cover the landfill and perform for a period of time. It, however, does not disclose an insoluble binder composition.
Accordingly from the foregoing, one object of the present invention is to provide a tackifier or binder composition as a dry mixture that dissolves readily only once to allow for application by spraying and thereafter becomes insoluble upon drying.
Another object of the invention is to provide a tackifier or binder after application and curing whose rate of biological decay is retarded but still ultimately decays into harmless products.
Still another object is to provide the tackifying material as a dry powder to an application site where it is then mixed with the application fluid thereby avoiding heavy drums of liquid compounds.
Still a further object is to provide a liquid tackifying application system that is a highly viscous solution reducing draining through a bed of mulch.
Yet another object is to provide a premixed fiber/binder system where the bonded fiber after application and drying has a high degree of resistance to wind and rain erosion.
Still a further object is to eliminate the use of preformed erosion-control blankets by providing a low-cost, one-step spray system.
These and other objects of the present invention will become readily apparent upon reading the specification to follow.